Most modern commercial and military transport airplanes are powered by turbo-fan jet engines suspended from the wings of the airplane by pylons. One end of the pylon is attached to the engine and the other end of the pylon is attached to the wing. The engine end of the pylon passes through the engine nacelle and attaches to several locations on the engine. The principal attachment locations are usually to the engine core or engine fan case.
The pylon is attached to the engine because the engine is heavy, and produces large static and dynamic as well as torque forces. As a result, the attachment locations must be capable of reacting (i.e., resisting) large forces. Because the engine nacelle is made of a lightweight material, incapable of withstanding large forces, the pylon attachment locations must be located within the nacelle's interior on the engine where large forces can be withstood.
While previously developed pylons have been satisfactory, they have several disadvantages that have become increasingly significant as turbo-fan engines have been improved through high by-pass turbo-fan engines to more recently developed ultra-high by-pass turbo-fan engines. The principal disadvantage of pylons is the requirement that a portion of the pylon extend into the core compartment area of the engine and partially block the flow of air through the fan duct of the engine. The portion of the pylon that blocks the flow of air is commonly referred to as the bifurcation, because it divides, or bifurcates, the air-flow through the fan duct. The air blocked by the bifurcation portion decreases the performance of the engine.
Thus, one way to improve the performance of turbo-fan jet engines is to reduce the size of the portion of the pylon that blocks air flow through the engine, i.e., the size of the bifurcation. Because of the static and dynamic forces that the pylon must withstand, in the past it has not been possible to reduce this portion of the pylon below a minimum value.
Another disadvantage of prior art pylons is the time required to perform maintenance tasks on the engine. In the past, many of the cables, hoses, ducts, etc., (hereafter collectively referred to as conduits) must be routed from the engine to other parts of the airplane through small openings in the pylon. This is done to prevent the conduits from creating additional blockage of the airflow through the engine. Many small passages, rather than one large one, are formed to avoid substantially weakening the structure of the pylon. The disadvantage of many small passages is that they are difficult for personnel to readily access when performing engine maintenance or repairs. Thus, maintenance and repair time for the engine is substantially greater than it would be if the conduits that pass through prior art pylons were more accessible.
A related disadvantage of prior art pylons is that they must be made of metal. Metal is required because some of the engine conduits that pass through prior art pylons carry high temperature fluids, such as engine bleed air, while others carry flammable fluids. Further, many prior art pylons include cavities to save weight. If a conduit or hose carrying a flammable fluid bursts, a cavity may fill with the flammable fluid and be ignited if the cavity is close to a conduit carrying a high temperature fluid or gas. Because structures formed of composite materials, which contain resins, are more likely to be damaged by heat than structures formed of metal, in the past pylons have been made of metal. Because pylons are expensive and because pylon heat damage is not uncommon, the ability to repair a pylon has been an important consideration when designing prior art pylons.